Concepts aimed at reducing fuel consumption and pollutant emissions are being introduced to an increasing extent in modern motor vehicles. These concepts include, inter alia, the implementation of what is called an automatic stop-start device, hereinafter referred to for simplicity as automatic stop-start, by means of which an internal combustion engine driving the motor vehicle can be switched off and started again independently of an intervention by the driver of the motor vehicle. In this case the internal combustion engine is switched off in particular in relatively long idle running phases in which the driving power of the internal combustion engine is not required. In this way considerable savings in fuel consumption can be achieved, in particular in urban traffic conditions.
However, the switching-off of the internal combustion engine by the automatic stop-start function is subject to restrictions. For example, it makes sense for reasons of comfort and economy not to allow the internal combustion engine to be switched off by the automatic stop-start in all cases, but rather to prevent this in certain traffic and driving situations, for example in stop-and-go operation (repeated stopping with short driving distance between, in a traffic jam for example).
DE 44 12 438 C1 discloses a method wherein in a first method step the running combustion engine is automatically switched off via an electronic control device by interruption of the fuel supply if specific engine stop conditions are fulfilled for the duration of a predetermined holding time. In a second method step the previously stopped combustion engine is restarted automatically by means of the control device if specific engine start conditions are fulfilled. One of the engine stop conditions is that the traveling speed of the motor vehicle is below a predetermined limit value, is preferably zero, and the rotational speed of the combustion engine with the drive clutch closed is below a predetermined value. The engine start conditions includes at least that no gear is engaged and the clutch pedal is pressed, or that no gear is engaged and the engine temperature has risen above a predetermined value.
Many of the known automatic stop-start systems can be manually deactivated by the driver of the vehicle equipped with the internal combustion engine, as a result of which the potential in terms of possible fuel saving is not or at least not fully exploited.
In a motor vehicle that is equipped with a gearbox that is to be shifted manually or with an automated manual transmission, minimizing the fuel consumption or, as the case may be, the polluting emissions of the motor vehicle requires the driver to make a gear change at the right time as a function of the operating point of the internal combustion engine. This applies in particular when shifting into a higher gear.
In this respect it is known to determine the optimal time for a gear change and communicate this to the driver, by means of a display on the instrument panel for example.
U.S. Pat. No. 4,555,691 discloses defining, or, as the case may be, determining, the shifting point as a result of reaching an engine speed threshold and a vehicle speed threshold.
U.S. Pat. No. 4,539,868 describes a method wherein initially the current engine operating point characterized by engine load and engine rotational speed is calculated. Said operating point is placed into the consumption characteristic map of the engine, whereupon a comparison with the consumption optimum is carried out. If a better consumption is possible, a lamp is lit up on the instrument panel.
A similar method is known from DE 31 28 080 A1, wherein after comparison of operating data with a stored consumption characteristic map the controller outputs a signal to a gear shift display when the stored limit values are chosen such that when the limit values are exceeded at least an equally great tractive force with a greater throttle valve opening is available at the drive wheels in the next higher gear.
The driver of the motor vehicle can follow a gear shift recommendation of this kind regarding an optimal shifting time or can also ignore it, as a result of which the potential in terms of possible fuel saving is not or at least not fully exploited.
Hybrid drives are known as a further possibility of lowering the fuel consumption and harmful emissions of a motor vehicle. They represent a combination of at least two different drive systems, usually a combination of a combustion engine with an electric motor. The aim of the hybrid drive is e.g. to use the pollutant-free electric drive first and foremost in urban traffic and to use the combustion engine in long-distance traffic and on interstates. In this case too a preferably visual and/or audible gearshift recommendation can be output to the driver of the motor vehicle with the aim of prompting him to switch manually from one mode of operation to the other mode of operation.
The time of the recommendation to shift to a mode of operation that is optimal in terms of fuel consumption and pollutant emissions can be transmitted wirelessly into the motor vehicle for example by means of signals from a stationary traffic installation within the infrastructure, e.g. when the vehicle is being driven into the pollutant-limited zone of a city center.
What has already been said above also applies here, namely that the driver of the motor vehicle can follow or ignore said shift recommendation. In the latter case valuable potential for saving fuel and lowering emissions is lost.
The same statements apply to what is termed a bivalent motor vehicle that is driven by means of an internal combustion engine which can be powered by two different types of fuel and produces different exhaust gas emissions as a function of the currently selected fuel type. Thus, in the case of an internal combustion engine that can be powered optionally by means of natural gas (gaseous (CNG) or liquid (LPG)) or by means of gasoline, for example, the pollutant emissions during operation by means of natural gas are much lower owing to its varietal purity and its high hydrogen content than during operation using gasoline fuel.
In addition to an automatic switchover between the two fuel types triggered by a control device, when for example one type of fuel is running low, it is usually also possible for the driver of such a motor vehicle to be able to switch between the two modes of operation simply by pressing a button. Similarly, as already mentioned above, a shift recommendation, which the driver can once again implement or ignore, can be output, when the vehicle is entering a pollutant-limited zone of a city center for example.